Label Distribution Protocol (LDP) enables peer label switch routers (LSRs) in a Multi-Protocol Label Switching (MPLS) network to exchange label binding information for supporting hop-by-hop forwarding. Various Layer 2 services (such as Frame Relay, Asynchronous Transfer Mode, Ethernet and the like) may be emulated over an MPLS backbone by encapsulating the Layer 2 Protocol Data Units (PDU) and transmitting them over a pseudowire (PW).
Typical VPLS (Virtual Private LAN Service) scenarios call for a full mesh of Pseudo-Wires between participating provider edge (PE) nodes, which is achieved using a full mesh of targeted LDP (T-LDP) session connectivity. This requires each PE node to maintain a large number of T-LDP sessions, which is challenging in High Availability scenarios since the cost of maintaining TCP sessions is very high.